This invention relates generally to a feedback assembly for pneumatic transmitters, and more particularly to a range spring sub-assembly for the feedback assembly to provide a variable gradient with minimum hysteresis.
In the copending application of Satori, Ser. No. 703,529, filed July 8, 1976, entitled "Pneumatic Transmitter for Variable Area Flowmeter," issued as U.S. Pat. No. 4,019,369, whose entire disclosure is incorporated herein by reference, there is disclosed a pneumatic transmitter which generates a pneumatic output signal proportional to a metered fluid flow rate.
The fluid is metered by a variable area flowmeter having a float therein that rises and falls, the float movement being converted by a magnetic follower into a corresponding rotary movement. The pneumatic transmitter is coupled to the magnetic follower, use being made for this purpose of an edge-profiled disc mounted on the follower shaft. The disc rotates in a direction and to an extent depending on the rise and fall of the float, the disc movement therefor being a function of flow rate.
Associated with the disc is an edge sensor constituted by a jet nozzle emitting a stream of air that is picked up by a jet collector, the edge of the rotary disc being inserted in the gap between the nozzle and the collector. Accordingly, the air pressure recovered by the collector increases as the edge profiled disc withdraws from the air stream and decreases as the disc edge moves toward the stream.
To translate the metered flow rate into a corresponding pneumatic signal, the edge sensor is mounted on the pivoted feedback arm of a feedback assembly. The arm is actuated by a feedback bellows responsive to the collector output of the edge sensor. Thus as the collector output pressure changes, the bellows acts to swing the pivoted arm in a direction and to an extent causing the edge sensor carried on the arm to assume a position with respect to the edge of the disc at which the pressure balance of the system is restored. The pressure applied to the bellows at equilibrium represents the pneumatic output of the system.
In the Satori feedback assembly, bridged between the pivoted feedback arm carrying the edge sensor and a fixed arm is the tension spring of a range spring sub-assembly which includes a zero adjustment knob and a span adjusting screw whereby the span of the system may be adjusted as desired. This sub-assembly is subject to hysteresis and gives rise to a pneumatic output signal which is not accurately proportional to changes in input.